反转链表

来源:互联网 发布:node.js 安装 编辑:程序博客网 时间:2024/03/29 15:29

输入一个链表,反转链表后,返回新的头节点


class ListNode {    int val;    ListNode next = null;    ListNode(int val) {        this.val = val;    }}/** * 方法1:先cur.next = pre,再递归reverse(cur, cur'.next),同时上传新的头节点 */public class Solution {    public ListNode ReverseList(ListNode head) {        if(head == null) return null;        ListNode newHead = reverse(head, head.next);        head.next = null;        return newHead;    }    public static ListNode reverse(ListNode pre, ListNode now){        if(now == null) return pre;        ListNode temp = now.next;        now.next = pre;        return reverse(now ,temp);    }}/** * 方法2:方法1的while循环版本,cur.next = pre, cur = cur'.next */public class Solution {    public ListNode ReverseList(ListNode head) {        if (head == null) return null;        ListNode cur = head.next, pre = head, temp;        head.next = null;        while (cur != null) {            temp = cur.next;            cur.next = pre;            pre = cur;            cur = temp;        }        return pre;    }}/** * 方法3:先递归reverse(cur.next),再cur.next.next = cur,同时将新头节点上传 */public class Solution {    public ListNode ReverseList(ListNode head) {        if(head==null || head.next == null) return head;        ListNode newHead = ReverseList(head.next);        head.next.next = head;        head.next = null;        return newHead;    }}


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